A conventional solid state fuel cell comprises a solid state electrolyte, a solid state cathode, and a cermet anode. The operating temperature of a stack of solid state cells is approximately 1000.degree. C. The electrolyte contains usually a Y.sub.2 O.sub.3 --ZrO.sub.2 mixture oxide, a so called yttrium oxide stabilized zirconium oxide (YSZ). The cathode is usually composed of LaMnO.sub.3 doped with Sr, Mg or Ca, and the anode is usually composed of a mixture of fine Ni-particles and YSZ. The nickel-YSZ-oxide cermet anode is usually produced by mixing NiO with YSZ (Zr.sub.1 -x Y.sub.x O.sub.2 --x/2). The latter oxide mixture is sintered on the electrolyte. When the fuel cell operates, and H.sub.2 or CH.sub.4 is transferred to the anode, NiO is reduced to Ni. Such a cermet anode must be porous in such a manner that the fuel gas can escape and react with O.sup.-- -ions from the YSZ electrolyte at the same time as it releases electrons to the nickel metal. Accordingly, the anode reaction can only take place in a transition area between the three phases YSZ, Ni, and fuel gas. The anode reaction is as follows: EQU CH.sub.4 (gas)+4O.sup.-- (YSZ electrolyte).fwdarw.CO.sub.2 (gas)+8e.sup.31 (in the Ni metal).
A conductive path or percolation path must be provided through the Ni-phase in order to allow removal of the electrons, the path being ensured by the % by volume of Ni exceeding 35%.
However, when CeO.sub.2 -based anodes are used, a reduction by way of H.sub.2 or CH.sub.4 results in formation of understoichiometric CeO.sub.2-x capable of conducting both electrons and oxygen ions. An oxidation of H.sub.2 or CH.sub.4 can on such an anode be carried out on the entire surface, i.e. the gas transition zone, e.sup.- and O.sup.-- simultaneously being available everywhere on the surface. The latter is of particular importance in connection with oxidation of CH.sub.4 presenting the main component in natural gas. The surface density, i.e. the reaction rate, in connection with understoichiometric CeO.sub.2-x is up to a 100 times higher than in connection with a Ni--YSZ cermet anode.
According to a conventional way of adhering a thin layer of ceramics, such as CeO.sub.2, onto another material, such as YSZ, the CeO.sub.2 powder is suspended in a suitable, viscous liquid spread over the already produced YSZ followed by heating to 1200.degree. to 1600.degree. C. for 1 to 5 hours. Subsequently, the CeO.sub.2 powder is sintered to an approximately compact layer capable of adhering to YSZ. During the heating, the CeO.sub.2 and YSZ diffuse into one another, CeO.sub.2 diffusing faster than YSZ, whereby porosities arise about the original YSZ surface due to the so called Kirkendall effect. The porosities reduce the mechanical strength with the result that the structure cracks. When the sintering temperature is kept low and the sintering period is short, the formation of porosities can indeed be reduced, but the sintering of the CeO.sub.2 electrode is, however, poor and results in a mechanically weak structure.
The diffusing of CeO.sub.2 into YSZ involves the additional problem that a poor electrical conductivity is achieved in the CeO.sub.2 --YSZ mixture area. Accordingly, a voltage drop arises in the area due to the high resistance.